// javascript is weekly typd
// Simple type system
// Number(float) ,String, Boolean,Date,Function,Array and Object
// Special types
// NaN,null,undefined
// Not a number 检查是否是一个合法数字
// null and undefined is differenct 


//typeof opeator
//return a string of the data type primitive

//instanceof operator
//return a boolean of a value matches the data type
// array is special object

let  x = "something";

console.log(typeof x);

x = 1;
console.log(typeof x);
x = 1 + "hello";
console.log(typeof x)
// constructor create is true
console.log(x instanceof String)

let y = 0=='' // 0 -> false '' -> false
let z = 0===''
